Service – performance and seakeeping trials on a car-ferry

Abstract

The present analysis of performance data, gathered on the car-ferry m.s. ROI BAUDOUIN, has been largely based on the properties of the apparent slip. The assumption of invariability of wake in this special case of a twin-screw vessel enables the authors to map out a generalized power diagram, that remains unaffected by fouling of the hull. All available performance data, gathered by only one experimenter on board, and thus not quite simultaneous, could be converted to values related to a standard condition of loading and hull surface. These values are used to map the iso-weather curves in the delivered horsepower/ship speed diagram. For that purpose, however, an ‘equivalent Beaufort number’ had to be introduced, i.e. an index of total influence on added power at a given ship speed by wave height and wind strength, which actually are not simply related to each other in coastal waters.

Seakeeping qualities have been investigated also. Waves, pitching and heaving motions, vertical and transversal accelerations, maindeck stresses amidships have been measured. Responses of ship motions, vertical accelerations, relative vertical motions and velocities, bending moments to regular headwaves have been computed theoretically and, on the basis of the measured spectra, the statistical values calculated. The agreement with the measured values is satisfactory, except for vertical bow accelerations. The theoretical responses, though, were considered as being sufficiently accurate to investigate the seakeeping in severe sea states. A prediction of excessive bow accelerations, slamming, shipping of water, propeller racing and main deck stresses was attempted. It could be concluded that excessive vertical accelerations at the fore-end of the garage is the main cause of voluntary speed reduction. A criterion of maximum allowable bow acceleration, by which the attainable speed can be determined in severe head seas is suggested. In this way, it is pointed out that for this car-ferry no voluntary speed reduction is necessary up to a sea state Beaufort 7-8 in the coastal waters under consideration.